This invention relates to semiconductor laser apparatus, and in particular to a means of achieving a highly stable laser output frequency.
At the present time, lightwave communications systems generally employ a base band amplitude-shift-keyed operation. That is, the laser is turned on and off so that the presence of a pulse constitutes a binary "1" and the absence of a pulse constitutes a binary "0". In such systems, even single-mode lasers are not truly coherent since turning the device on and off will vary the frequency.
While such systems are generally adequate for present communication systems, several advantages would accrue if a coherent system were employed. In such a system the laser carrier frequency is well controlled and the information is impressed on the carrier by changes in the frequency, phase or amplitude of the light signal. For example, higher capacity wavelength division multiplexing could be employed due to the well-defined frequency of each channel. In addition, a highly stable local oscillator may be used to implement a homodyne or heterodyne system resulting in greater receiver sensivitity and increasing the permissible spacing between repeaters.
In the semiconductor laser art, it is known to employ a Fabry-Perot interferometer to detect changes in the laser frequency and to use this signal in a feedback loop to control the temperature or drive current of the laser (See, e.g., Tsuchida et al, "Improvements in the Short-term Frequency Stability of AlGaAs DH Laser," Transactions of IECE of Japan, Vol. E65, pp. 65-66 (January 1982).) However, as far as applicants are aware, a high degree of stability in prior art methods was achieved only by very complex electronic feedback loops.
In gas laser systems, it has also been proposed to employ a Fabry-Perot interferometer in a double feedback loop to control the length of the laser cavity by coupling one loop to a piezoelectric device and the other loop to a heater. (See Baer et al, "Frequency Stabilization of a 0.633 .mu.m He--Ne Longitudinal Zeeman Laser," Applied Optics, Vol. 19, pp. 3173-3177 (September 1980).)
It is a primary object of the invention to provide a feedback system for producing a highly stable output frequency from a semiconductor laser.